1. Field of the Invention
The present invention relates to pager having battery saving means.
2. Description of the Prior Art
FIG. 2 shows signals of the POCSAG-type (Post Office Code Standardization Advisory Group) which are used in pager systems. The signal comprises a preamble .alpha. and plural batches .gamma.1, .gamma.2 . . . which follow the preamble .alpha.. Each batch comprises a frame synchronizing signal S1, S2 . . . and several frames (e.g. eight frames of #0 to #7). The preamble .alpha. indicates that batches .gamma.1, .gamma.2 . . . will be sent following its preamble .alpha., and the preamble .alpha. comprises repetitive signals of 1's and 0's. In the batch, respective frames are sent after respective predetermined time periods from the frame synchronizing signal. Each pager has a code with a group number (e.g. one of #0 to #7) assigned thereto and an individual address code within said group.
FIG. 2(b) shows the operation of the Prior-Art pager. Assume that the pager is assigned with a group number of #2, and assigned an individual address code of #11506. The pager watches for the preamble .alpha. in watching operation .eta.. When the pager does not receive the preamble .alpha. in the examining operation .delta.1 for the preamble, the pager turns off a battery to save battery power (battery saving operation). After a predetermined time, the pager turns on the battery again to carry out another examining operation .delta.2.
In an examining operation .delta.3, the pager receives preamble .alpha. from the base station. Then the pager maintains the ON-state of the battery, watching for the frame synchronizing signal S (carrying out a watching operation .epsilon. for the frame synchronizing signal (see FIG. 2(b)). After receiving the frame synchronizing signal S1, the pager turns off a battery and maintains the OFF-state of the battery until reaching the selected frame #2 which corresponds to its group number #2. When the predetermined time period for the selected frame (#2) comes, the pager turns on the battery. The pager turns on the battery a little before the beginning time of the frame #2 to ensure reception of the entire frame #2.
This pager (group #2, individual address #11506) receives information signals which follow the individual address signal corresponding to its individual address code #11506. In the frame #2 of the 1st batch .gamma.1, the pager carries out an examining operation .zeta.1 for examining whether the received individual address code corresponds with its individual address code (#11506) or not. In this case, when the received individual address code (#12002 see FIG. 2(a)) does not correspond to the pager's own individual address code (#11506), the pager turns off the battery (see FIG. 2(b)) after the examining operation .zeta.1. Therefore, the pager does not receive information signals which follow the individual address signals, because the information signals must be intended for another pager (#12002).
In the frame #2 of the 3rd batch .gamma.3 (examining operation .zeta.3), the pager finds an individual address signal corresponding to its individual code (#11506). Then the pager maintains the ON-state of the battery, and receives information signals intended for it.
When the frame synchronizing signals are not received, the pager carries out another watching operation in the next preamble which will come later.
FIG. 1 shows a block diagram of the prior art pager. In the examining operation .zeta.1, the signal from the base station is received through an antenna and demodulated into digital signals by a receiving circuit 2. The digital signal is rectified by a rectifier circuit 3. This rectified digital signal is synchronized with a reference clock signal from a clock generating circuit 11 by a bit synchronizing circuit 4 (the synchronization establishment operation). This synchronized digital signal (hereinafter the rectified signal) is inputted to an examining circuit 6. The examining circuit 6 examines whether the rectified signal is the preamble signal or not by comparing the rectified signal with preamble data generated in a circuit 5. Since in the examining operation .zeta.1 the preamble signal is not detected, a battery saving circuit 12 stops supplying power from the battery 16 to the receiving circuit 2 and the rectifier circuit 3. Then the pager maintains a watching operation .eta. for the preamble.
Since in the examining operation .zeta.3 the preamble signal is detected, the battery saving circuit 12 does not stop supplying power from the battery 16. Then, the pager observes the frame synchronizing signal S1 with the examining circuit 8, which compares the rectified signal and frame synchronizing data generated by circuit 7. When the frame synchronizing signal is detected, the battery saving circuit 12 stops supplying power until frame #2 arrives. In frame #2, an examining circuit 10 for examining individual address codes compares the rectified signal and the individual address code generated in circuit 9, and detects the individual addressing. When the examining circuit 10 detects the individual addressing, the speaker 15 is energized to make a beep tone by a buffer 14 and a control circuit 13.
The above-mentioned Prior-Art pager has disadvantages as follows.
When the pager fails to detect the preamble .alpha. in the examining operation .zeta.3, the pager fails to receive the information signal. By failing to detect the preamble due to fading etc., the pager maintains the watching operation n as shown in FIG. 2(c). The preamble .alpha. will not be detected in examining operations .zeta.4, .zeta.5 and .zeta.6. Therefore, the pager cannot carry out the examining operations .zeta.1, .zeta.2 and .zeta.3, and it fails to receive the information signals. In the prior art apparatus, bit-synchronization must be established prior to the preamble examination. The time required for such a preamble examination necessitates a longer time than the sum of the time required to establish synchronization in the bit synchronizing circuit 4 and the time required for preamble pattern examining by the examining circuit 6. Accordingly, there is a problem of taking too long for preamble examining.